The present invention relates in general to voltage regulators and, more particularly, to a DC-DC current-mode controlled flyback switching power supply.
The use of switching-type power supplies in residential and commercial applications for DC-DC voltage conversion is highly desirable due to advantages, such as increased efficiency, decreased size, and decreased weight of the switching supply over traditional linear-type power supplies. Switching power supplies become more cost effective in comparison to an equivalent linear power supplies as the electrical load power requirement increases above say 20 watts.
In general, the switching power supply converts an unregulated DC input voltage to the desired DC output voltage. A rectifier stage may be included prior to the switching power supply to convert an AC voltage into the unregulated DC input voltage. The input voltages for switching supply conversion typically range from 90 to 130 volts AC in the United States, Canada, and Japan; 220 to 240 volts AC in Europe; and 400 to 600 volts AC for worldwide industrial applications. The AC voltages once rectified correspond to DC voltages ranging from 100 to 850 volts DC. The switching power supply produces DC output voltages of say 24 volts or less.
To accommodate the wide range of input voltages, the prior art has used a variety of power supply designs to achieve optimum performance. For example, forward-mode switching converter supplies are typically used to accommodate input voltages with the range of 90 to 130 volts AC. It is well known in the art that for a forward-mode converter with a fixed load, as input line voltage increases, the duty cycle of the switching converter can decrease to a value as small as 3%. In addition, a large power switching transistor, with characteristically large junction capacitance, is utilized to facilitate the load supply current. Since the transistor is typically switching at a rate of 100 kilohertz, the low duty cycle will not permit the transistor to turn-on completely, thus resulting in loss of efficient operation.
The boost-mode flyback power supply optimizes power conversion for larger ranges of input line voltages encompassing from 90 to 240 volts AC. It is known that boost-mode supplies are limited in operating voltage range due to circuit limitations imposed by core related energy transfer requirements. It is also known, for example, a boost mode flyback supply designed to operate with a 50% duty cycle at 90 volts AC experiences deterioration to a duty cycle of 3% at 240 volts AC resulting in a significant loss of efficiency. Furthermore is understood that the increased input voltage demands of industrial power supply converters, 280 to 600 volts AC, require major circuit design modifications from typical lower voltage forward-mode and boost-mode flyback supply techniques to assure efficient operation for the higher range of operating voltages. Still another problem that exists for DC-DC converters is the performance variation caused by load variations.
Hence, a need exists for an efficient DC-DC switching power supply design that is capable of operating throughout a wide range of input voltages.